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.Dd March 18, 2012
.Dt MSYNC 2
.Os
.Sh NAME
.Nm msync
.Nd synchronize a mapped region
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/mman.h
.Ft int
.Fn msync "void *addr" "size_t len" "int flags"
.Sh DESCRIPTION
The
.Fn msync
system call
writes any modified pages back to the file system and updates
the file modification time.
If
.Fa len
is 0, all modified pages within the region containing
.Fa addr
will be flushed;
if
.Fa len
is non-zero, only those pages containing
.Fa addr
and
.Fa len-1
succeeding locations will be examined.
The
.Fa flags
argument may be specified as follows:
.Pp
.Bl -tag -width ".Dv MS_INVALIDATE" -compact
.It Dv MS_ASYNC
Return immediately
.It Dv MS_SYNC
Perform synchronous writes
.It Dv MS_INVALIDATE
Invalidate all cached data
.El
.Sh RETURN VALUES
.Rv -std msync
.Sh ERRORS
The
.Fn msync
system call
will fail if:
.Bl -tag -width Er
.It Bq Er EBUSY
Some or all of the pages in the specified region are locked and
.Dv MS_INVALIDATE
is specified.
.It Bq Er EINVAL
The
.Fa addr
argument
is not a multiple of the hardware page size.
.It Bq Er ENOMEM
The addresses in the range starting at
.Fa addr
and continuing for
.Fa len
bytes are outside the range allowed for the address space of a
process or specify one or more pages that are not mapped.
.It Bq Er EINVAL
The
.Fa flags
argument
was both MS_ASYNC and MS_INVALIDATE.
Only one of these flags is allowed.
.It Bq Er EIO
An error occurred while writing at least one of the pages in
the specified region.
.El
.Sh SEE ALSO
.Xr madvise 2 ,
.Xr mincore 2 ,
.Xr mlock 2 ,
.Xr mprotect 2 ,
.Xr munmap 2
.Sh HISTORY
The
.Fn msync
system call first appeared in
.Bx 4.4 .
.Sh BUGS
The
.Fn msync
system call is usually not needed since
.Bx
implements a coherent file system buffer cache.
However, it may be used to associate dirty VM pages with file system
buffers and thus cause them to be flushed to physical media sooner
rather than later.
